Crosslinked polyolefin foams are produced commercially worldwide, playing a part in numerous industries. They can be found for example in sports and leisure products, in military applications, in vehicles, such as ships and boats and in the home.
Although naturally occurring polymer foams have been known for a long time, (e.g., sponges, cork), synthetic polymer foams have only been introduced to the market over the last 3 decades.
The production of cross-linked polyolefin foams involves the cross linking followed by foaming. In the cross linking step covalent bonds are formed between the polymer creating a three dimensional macroscopic matrix, which provides the material both physical and chemical strength and robustness. In the foaming step, a blowing agent is activated to release gas which is entrapped within the cell units of the matrix, thus forming small “air bubbles”.
Electron beam, silane grafting and peroxides are the main three techniques used for crosslinking of polyolefin foams. Typically, for electron beam irradiation, an extruded polymer sheet containing the chemical foaming (blowing) agent is subjected to intense electron beam energy which serves to dehydrogenate the polymer, an action that results in crosslinking. The polymer is heated further to activate the foaming agent. For chemically crosslinked polymers (e.g. polyolefins), the crosslinking agent, usually peroxide, is compounded into the polymer along with the foaming agent. The extruded polymer sheet temperature is sufficient to crosslink the polymer but not to activate the foaming agent. After most of the crosslinking is completed, the polymer sheet is heated further to activate the foaming agents. An organosilane and a free radical generating compound reacted with a polyolefin forms a silane grafted polyolefin that when mixed with a silanol condensation catalyst and heated, creates a silane crosslinked polyolefin. This crosslinked polyolefin can then be subsequently foamed using a conventional chemical blowing agent that has been previously compounded into the sheet.
The combination of naturally occurring polymer foams and synthetic polymer foams has also been suggested. The physical properties of cork, a natural polymeric foam, was already appreciated by ancient Egyptians who used cork to prepare fishing boats, taking advantage of the fact that a significant amount of a cell unit of cork (usually more than half) consists of air, this made cork one of the lightest substances available at the time. In addition to its lightness, cork is a material exhibiting excellent compressibility, resilience, moisture resistance, high friction resistance, and other advantageous qualities.
Cork is utilized as a raw material for the production of a variety of products including as an ecological insulator in constructions, in sheets with vibration absorbance to be used e.g. in machinery, in soles (in footwear), in stationery, leatherwear, packing, floor covering (laminates) and stoppers for wine bottles.
In one aspect, cork is used as an additive filler particle in the production of utility polymers. Since the cork particles are chemically inert in a cross linking reaction, they may act as nucleating agents, and thereby tend to promote formation of air bubbles. The cork particles are added in the form of powder, pellets, granules or chips which may be of regular or irregular shape.
Examples for the various utilizations of cork are described in the following publications: in GB 1347797 ground cork is suggested as an additive to a cushioning polymer. In FR 2866591 sheet material is made from particles of a granulated material, including cork, coated with a bonding agent of a polymer, rubber or other material that can be vulcanized. In JP 2004058557 cork grains are added to a mixture of polyamide fibers, a polymer and charcoal powder to form a flexible sheet; In JP 2000043012 cork sheet is provided comprising cork powder and an olefin copolymer; In JP 60034680 floor covering is described comprising cork powder mixed with crosslinkable rubber or thermoplastic rubber like polymer; in GB823765 a resilient material for the manufacture of pads and supports are prepared by calendering into sheet form, and subsequently vulcanizing, a mix containing particles or granules of, inter alia, cork, and a bonding medium, such as synthetic or natural rubber; In JP 2001213990 there is described a copolymer of ethylene and C3-12 α-olefin mixed with up to 10% w/w cork powder providing particles In U.S. Pat. No. 4,743,636 there is provided a condensed (non-foam) polymer based on low density PE (LDPE) and linear LDPE which are mixed with filler particles, including cork particles in order to make a sealing compound.